let N be Pnet; :: thesis: for x being Element of Elements N holds Post (N,x) c= Elements N
let x be Element of Elements N; :: thesis: Post (N,x) c= Elements N
for y being object st y in Post (N,x) holds
y in Elements N by Def7;
hence Post (N,x) c= Elements N by TARSKI:def 3; :: thesis: verum